This invention relates to a method for providing conductive paths from the outer surface of conductor insulating jackets to the magnetic cores in dynamoelectric machines for purposes of inhibiting corona in the machine.
In large dynamoelectric machines, such as the generators used by electric utilities, the high voltage winding has the sides of the coils located in slots in a magnetic core member. The coils used in these machines have jackets for insulating the conductors from the core. Since these machines operate at relatively high voltage, the outer surface of the jacket usually includes a covering of a semiconducting material serving as a voltage grading medium and commonly known as the coil armor. The jackets usually consist of wrappings of porous materials impregnated with certain thermosetting resins which cure to a solid and hard state. The sides of the coils, i.e., those portions of each coil to be placed in slots in the core, are shaped in molds while the resins are still plastic and held to these shapes while the resins cure. The outer surfaces of the coil side are left very smooth, hard, and with some irregularities in flatness. To facilitate the insertion of the coil sides into the respective slots, each side is usually made to be a clearance fit in its slot. Because the slots may vary somewhat in size and surface smoothness, the sides are made to tolerances which take this into account. As a result, there is usually some looseness of a side in its slot. It is known to insert packing strips between a coil side and a slot wall to tighten up the fit and thereby prevent movement of the side in the slot. These strips may be thin, non-metallic, electrically conductive springs which secure the side in the slot and provide electrical paths of controlled resistance between the coil armor and the slot wall. However, since the strips come in discrete thicknesses that can be driven between a coil side and a slot wall, this packing may not always make a side a tight fit in all of its slot, particularly if the winder is lacking in skill or is not careful; any looseness may lead to coil movement resulting in corona problems.
Electrical grade resinous materials should be good insulators of electricity and reasonably good conductors of heat. Certain epoxy resins meet this specification. However, those that do meet the specification cure to a hard state, and once fully cured, they do not soften appreciably when reheated during operation of the machine. These materials produce the so-called hard-bar windings in which the resin impregnants do not soften, when the coils become hot, and flow into the voids as did the asphaltic impregnants that preceded them. Hence, the hard-bar windings are more prone to void formations due to the following: looseness of bars in their slots; irregularities in bar surfaces; irregularities in slot walls; and coil vibrations.
If the coil armor makes good electrical contact with the laminations defining the slot walls, the armor and the core will be at essentially the same potential. However, the presence of voids degrades these contracts and frequently leads to differences in potential between the armor and core. These potential differences impose electrical stresses on the gases in the voids, stresses that may well be great enough to cause partial discharge from the coil surfaces to the core, i.e., a phenomenon often referred to as corona or corona discharge. The improved resinous materials make higher operating voltages possible, and this in turn subjects the void regions to higher electrical stresses, or these newer insulations may even increase stresses without an increase in voltage. It is well known that in the presence of corona discharge insulating materials are eroded and may eventually break down.
Our copending Canadian application, Ser. No. 192,819, filed Feb. 18, 1974 describes and claims a means for inhibiting corona in dynamoelectric machines such as large power generators. In this application, an elastomeric material of controlled electrical resistance is applied to the coil sides and then cured before the sides are inserted into the slots in the core. The lay of this material on a coil side is such that the material deforms as the side is inserted into a slot, causing the material to make contact with the laminations. In this particular approach to the corona problem, the elastomeric material is applied to the coil sides before they are inserted in the slots; the material cannot be applied to the coil sides already in place in the slots. This mode of treatment produces good results, but in some machines, e.g., the stator core of a steam turbine-generator, it is difficult to insert the coils sides in the slots.
According to the invention conductive paths are formed between the winding and the core of a dynamoelectric machine by injecting a viscous, semiconducting, elastomeric material between the coil sides and the walls of the coil retaining slots in the core through the use of an injector tool inserted between them, and thereafter curing the material. The cured material is a tough rubber-like substance of an electrical resistance high enough not to short circuit the laminations of the core and yet low enough to conduct electric charge from the coil armor to the core; it is a substance that is capable of retaining its strength, elasticity, conductivity, etc., and remaining in place between the coils and core under vibration coolant flow, electric stresses, repeated temperature changes, etc., for the normal operating life of the machine. These paths conduct electric charge from the coils to the core, and thereby inhibit the formation of corona. Resin flow into the coolant ducts is blocked off during injection.
Certain silicone resins are well suited for injection as conductive path forming materials between the coils and core. Inherently, silicone polymers, i.e., rubbers, are good electrical insulators, but some are relatively good conductors of heat as well. The good heat conductors are preferred because they will transfer heat from the coils to the core. To make them electrically conductive for purposes of conducting electric charge, they are filled with conductive fine particle materials such as carbon powder, lamp black or a mixture thereof. The amount of conductive power added to the resin is just enough to give the cured product (the rubber) the necessary electrical properties, but not enough to detract significantly from its physical properties.
Apparatus for injecting an uncured elastomeric material into the space between a coil side and a wall of the slot containing the side may consist of an injector tool adapted for insertion into this space and means for forcing the material to flow through the tool into the space. Means are also provided for blocking flow of the material into the coolant flow ducts on either side of the tool.
A preferred embodiment of the invention will now be described with reference to the accompanying drawings, in which: